Caps used with liquid containers in which withdrawal of the contents is to be achieved without removing the cap have been known for many years. Often such caps are used in medical or analytical scientific applications where a reliably sealed container, that also affords ready access to its liquid contents, is required.
One such example is shown in U.S. Pat. No. 1,180,665 which discloses a cap having a solid, cylindrically-shaped body made of resilient material with a shoulder which abuts the rim of the container opening. Since the opening is slightly smaller than the dimensions of the body of the cap, a tight seal is formed when the cap is inserted. The cap includes a longitudinal hole which is compressed closed when the cap is inserted into the container opening. To remove the liquid contents, a syringe needle is used to penetrate the cap through the hole. Due to the resilient nature of the cap material, a seal will form around the needle as it penetrates the cap. When withdrawing the liquid contents from this capped container, the needle must be lined up exactly with the hole in the cap, and secondly, a partial vacuum will be formed within the container as liquid is withdrawn into the needle.
Problems of the type described above become magnified when the stoppered containers are used for LC analysis, especially in an automatic sample withdrawal system. In LC analysis, accurate mixtures of liquid samples are stored in stoppered containers for subsequent injection into the chromatographic instrument. Generally the containers are placed side by side on an automated carousel which positions the sealed container under an automated syringe needle which withdraws the appropriate amount of sample for subsequent processing. The need for precison positioning of the needle involves costly equipment and adds complexity to the instrument design. The partial vacuum produced as liquid is withdrawn from the sealed container may cause more of one component of the precise mixture to evaporate than another due to the highly volatile nature of the liquids typically used, thereby changing the composition and producing an error in the LC analysis. Moreover, since the partial vacuum forms in the space above the liquid as liquid is withdrawn, a portion of the sample in the needle could be drawn back into the container as the needle opening traverses the vacuum space thereby altering the intended volume to be withdrawn and producing resultant analysis errors.
Another cap is shown in U.S. Pat. No. 1,413,703. This cap is a two-piece, crimp-on type, with one piece being a resilient septum that fits over the container opening and the other piece fitting over the septum and around the rim of the container. The two pieces are crimped together to securely seal the container opening. The septum is designed to be piercled by a needle for withdrawing the contents of the container; however, as was the case with the afore-mentioned patent, a seal is formed around the needle as it penetrates the septum resulting in composition and volume errors previously mentioned with respect to the creation of a partial vacuum within the container. Furthermore, the two-piece construction is burdensome for it requires special crimping tools to create a sealed closure and this cap is not easily removed once crimped to the container. Also the lateral dimensions of the cap exceed that of the container thereby limiting the number of containers which can be packed side-by-side as in an automated carousel.
A wide variety of other special purpose caps exists in the patent art as exemplified by U.S. Pat. Nos. 3,901,402 and 4,193,402. However, the caps disclosed in these patents do not overcome the difficulties associated with partial vacuums when a needle is inserted to withdraw the contents of the container because in each instance a seal is maintained around the needle as liquid is being withdrawn.
Thus it is apparent the need still exists for an inexpensive, easily manufactured cap that is readily insertable into and removable from a container and that has a large area septum for piercing with a needle that will not cause a partial vacuum during withdrawal of liquid by the needle and that is adaptable for use in LC analysis without contributing unacceptable errors.